Neurons receive information through synaptic connections on their dendrites. Dendrites span long distances and branch intricately to contact the appropriate synaptic partners. The size and complex patterns of dendrites pose unique cell biological challenges. Thus, cellular organelles need to traffic far from the soma to maintain dendritic structure and support synaptic function. Mitochondria - the cell's power plants - are found throughout neuronal dendrites. In addition to producing ATP, mitochondria take up Ca2+ and participate in neuronal signaling. The special importance of mitochondria to neurons is highlighted by the fact, that, while all cells contain mitochondria, mutations that affect their trafficking and function manifest specifically as diseases of the nervous system. In particular, these diseases frequently affect retinal ganglion cells (RGCs), the output neurons of the eye, and cause vision loss. Despite their importance, we know little about how dendritic mitochondria move through neurons, how they target specific regions within dendrites, and how their local function shapes and supports neural development and function. This proposal uses a multidisciplinary approach to address these questions in RGCs in the intact retina. A combination of genetic strategies will be used to simultaneously label RGC dendrites, synapses and mitochondria. Using static high resolution imaging and time-lapse microscopy, the distribution, development and dynamic interaction of these structures will be analyzed. Next, the hypothesis that synaptic activity guides the movements of mitochondria during development and locally controls their function will be tested in transgenic mice in which synaptic input to RGCs is modified in vivo. Bimolecular sensors have been developed and tested to monitor mitochondrial function dynamically in their natural environment using optical approaches. Finally, the specific contribution of mitochondria to dendritic and synaptic development and function will be tested using genetic techniques to selectively interfere with mitochondrial localization or sensitize them to laser-ablation. The consequences of these manipulations for RGC development will be assessed using live imaging and their impact on visual function will be evaluated using patch-clamp electrophysiology. Together, the proposed experiments will not only advance our understanding of the fundamental cell biology underlying retinal circuit development and function, but also provide insight into the mechanisms of a growing number of nervous system disorders - involving eye and brain - that are caused by mutations in mitochondrial genes (e.g. dominant optic atrophy) and/or associated with mitochondrial dysfunction (e.g. Parkinson's and Alzheimer's disease).